With the recent development of high-speed and large-capacity optical fiber communication systems, high-speed optical modulators using a waveguide type optical element such as an external modulator have been widely used.
Since a waveguide type optical device of which a substrate is formed of a material such as LiNbO3 (lithium niobate which is also abbreviated as LN) having an electro-optical effect can cause a variation in refractive index with a voltage variation, the waveguide type optical device is used as an optical modulator (generally referred to as LN optical modulator) or an optical switch.
Regarding the increase in speed of the optical fiber communication systems, speeds of 10 Gb/s to 40 Gb/s have been used and the development toward 100 Gb/s is being carried out. To realize 100 Gb/s using an LN optical modulator, as shown in FIG. 5, a method of modulating light beams by 25 Gb/s using four optical modulators 901 to 904, combining the outputs of the optical modulator 901 and the optical modulator 902, and the outputs of the optical modulator 903 and the optical modulator 904, respectively, to acquire modulated light beams of 50 Gb/s, and combining two modulated light beams of 50 Gb/s in a state where the relative angle difference between polarization planes is set to 90° by the use of polarization beam rotating means 905 to acquire a modulated light beam of 100 Gb/s into which the polarization beams of the modulated light of 50 Gb/s are combined has been suggested (for example, see PTL 1). This method has a merit that a modulation circuit corresponding to 25 Gb/s can be used and thus is considered as a promissing method.